The present invention relates to an inverter system capable of reducing switching loss and a control system for an electric vehicle using the inverter system. More particularly, the invention relates to an inverter system, suitable for a control system for a vehicle, which is able to improve the utilization factor of the battery voltage and provide high efficiency driving.
As a common method of driving a three-phase alternating current motor using sinusoidal wave currents provided by an inverter system, there is a known method of switching inverters using a three-phase pulse width modulation signal. On the other hand, a method of supplying sinusoidal wave currents while reducing the switching loss is disclosed in U.S. Pat. No. 4,321,663. This method has been referred to as a two-phase switching method in which the switching of one of the three-phases is stopped (an upper arm or a lower arm of the one phase of an inverter is set in the ON-state) and is controlled so that its line-to-line voltage becomes a sinusoidal wave voltage using the pulse width modulation signals of the other phases.
Employing this method has two advantages. The first advantage is that the switching loss can be reduced to 2/3 of the switching loss in the three-phase switching method since the frequency of switching is reduced. The second advantage is that the range of the output voltage of the inverter system allowing the sinusoidal wave output current to flow can be expanded by 15% when the voltage of the inverter power source is constant. Hereinafter, this effect is referred to as an improvement in the voltage utilization factor.
A method which can further reduce the switching loss to a greater extent than the former method is disclosed in Japanese Patent Application Laid-Open No.7-46855. That is, in a two-phase switching method, a phase in which a large current is flowing can be selected as a target phase for which the switching is stopped in accordance with a phase difference in the output voltage and the output current of the inverter system, since the switching loss increases with an increasing switching current. In order to generate a predetermined sinusoidal wave output voltage in the two-phase switching method, the ranges in which the switching can be stopped include a range from 30 degrees to 150 degrees and a range from 210 degrees to 330 degrees in the phase of the sinusoidal wave voltage. In general, as described in the former reference, the switching is stopped in a range from 60 degrees to 120 degrees and a range from 240 degrees to 300 degrees in the phase of the sinusoidal wave voltage. Therefore, the phase of the two-phase switching within these ranges is referred to as a reference phase.
With respect to the reference phase, the phases of the two-phase switching are shifted within the ranges from 30 degrees to 150 degrees and from 210 degrees to 330 degrees so that switching is stopped for a phase having the largest output current in absolute value. Therefore, in the method of the latter reference, the switching loss can be reduced since the phase having the lowest currents are used for the switching.
The method of the latter reference has a disadvantage in that, since it is necessary to detect or calculate a phase difference between the output voltage and the output current from the inverter in order to improve the voltage utilization factor and decrease the switching loss, the construction of a control method for generating the pulse width modulation signals becomes complex.
Further, in the method of the latter reference, it has been found that, in a case where the phase difference between the voltage and the current exceeds 60 degrees, a phase having a large current is switched when only a given section is considered.